Electronic devices and systems which use integrated circuits are many-fold and ubiquitous. From the early days of semiconductor chips having hundreds of transistors, today's Very Large Scale Integration (VLSI) type semiconductor chips may have billions of transistors, allowing for complex functionality to be provided on a single semiconductor chip.
The functionality provided by the semiconductor chip may often require that the semiconductor chip is in some way unique and identifiable. For example, such functionality may be required for chips involved in authentication procedures such as for payment cards or smart cards for the delivery of media content or more generally the “secure enclaves” provided in modern mobile devices that handle device specific authentication and/or data encryption and the like. Often the unique data are, or the data processing is, required to be secret or confidential (for example, where a particular cryptographic key is embedded within the semiconductor chip itself).
However, while there is a need for individualization, for reasons of cost an efficiency semiconductor chips are typically manufactured in bulk, with modern manufacturing process that strongly favour the production large numbers of identical chips. The most widely used methods of bulk manufacturing semiconductor devices (or chips), such as VLSI semiconductor devices, uses stepper machines and optical (UV) lithography. As semiconductor chips and their manufacture is well-known, further detail shall not be provided herein. However, more information on semiconductor chips and the manufacture thereof (in particular VLSI type chips) can be found at, for example, https://en.wikipedia.org/wiki/Very-large-scale_integration, the entire contents of which are incorporated herein by reference. This allows large numbers of identical semiconductor chips to be created from single wafers of substrate (such as silicon). As is well-known, lithographic techniques typically involve selectively removing areas of a resist, which initially covers a surface of the substrate. This enables creation of very small structures in the resist that can subsequently be transferred to the substrate material by further processing. Such further processing typically involves etching and/or deposition of further material. The resultant structures on the substrate implement electronic circuits that provide the functionality of the semiconductor chip.
The optical (UV) lithography that is typically used in bulk manufacture of such chips involves using a photosensitive “resist”, and a mask having a negative (or positive depending upon the resist mechanism) image of the circuits to be applied. Light (typically UV light) is shone through the mask onto the resist. The areas of the resist illuminated by the light are chemically altered such that they may be selectively removed using a further chemical process. Typically, the optical exposure changes the solubility of the resist, enabling selective removal of either the exposed or non-exposed regions of the resist by immersing the resist in a solvent (i.e. developing). This creates the very small structures in the resist. As optical lithography (or photolithography) is well-known, further detail shall not be provided herein. However, more information on optical lithography can be found at, for example, https://en.wikipedia.org/wiki/Photolithography the entire contents of which are incorporated herein by reference.
The masks used in this lithographic technique are costly to make and it is considered impractical to make individual masks for the purpose of embedding unique identification data in individual chips. Therefore, such unique identification data may be embedded into bulk manufactured chips using programmable ROM (such as PROM). This typically, involves selectively applying a high voltage across certain transistors of the chip, after initial manufacture, to “blow” (or otherwise cause the transistor to break down) thereby encoding one bit of data for each transistor. However, such a technique is vulnerable to hardware attacks involving an attacker using probes, electromagnetic radiation, chemical reactions etc. to try to determine the inner workings or embedded information of the hardware device, or enable further functionality present on the chip but not authorized for use. Indeed, an attacker may simply use various scanning devices on a given chip to map the circuits on the chip, and simply create a clone chip, which would have the same embedded data.
However, optical lithography is known to introduce variability in the manufactured chips due to variations in lithographic distortion. Also lithographic techniques in general are subject to manufacturing tolerances that produce variations in the manufactured chips due to thickness variations resulting from chemical mechanical polishing (CMP), unevenness in film deposition and so on.
It is known that these variations can result in bulk manufactured chips having individual “fingerprints”. An example of this is the Physical Unclonable Functions (PUF) described in “Power-Up SRAM State as an Identifying Fingerprint an Source of True Random Numbers”, D. E. Holcomb et al., IEEE Transactions on Computers, 58, (2009), pp 1198-1210, which is incorporated herein by reference in its entirety. These manufacturing variations leads to memory cells being biased towards “1” or “0” on power up in dependence on the variations. Therefore, after chip manufacture it is possible to test the chip in order to determine memory cells with a bias. The biases of these cells represent a unique identification (or “fingerprint”) for the chip. The identity can be reliably detected with the use of an error correcting code. The very high number of memory cells on a chip leads to essentially unique fingerprints. In essence, the fingerprint value may be used to both represent an identification and/or a secret key which is sometimes used to authenticate the identity.
Even if an attacker were to clone one of these chips, the uncontrollable manufacturing variations ensure that the cloned chip would have a different set of variations and therefore a different fingerprint. Of course, as a corollary, since the fingerprint of a chip is dependent on the uncontrollable manufacturing variations, the manufacturer cannot embed a pre-determined fingerprint onto a given chip.
Semiconductor chips can also be created using electron-beam lithography (or e-beam lithography). E-beam lithography involves scanning a focused beam of electrons to draw or write custom shapes on an electron-sensitive resist. As fine control of beam scanning is possible, there is no need to use a mask. Here, the electron beam changes the solubility of the resist, enabling selective removal of either the exposed or non-exposed regions of the resist by immersing the resist in a solvent. As e-beam lithography is well-known, further detail shall not be provided herein. However, more information on e-beam lithography can be found at, for example, http://en.wikipedia.org/wiki/Electron-beam_lithography, the entire contents of which are incorporated herein by reference. An example of creation of chips using electron beam lithography is by Mapper Lithography (see http://www.mapperlithography.com/). Unfortunately, whilst e-beam lithography can be used for producing short runs of semiconductor chips it tends to be unsuitable (due to cost and speed) for the bulk manufacture of semiconductor chips, especially VSLI type chips. In some system, such as those produced by Mapper Lithography, many individually controlled e-beams are used in parallel in an attempt to overcome this problem.